If f(x)=∑n=0∞anxnf(x) = \sum_{n=0}^{\infty} a_n x^nf(x)=∑n=0∞anxn and g(x)=∑n=0∞bnxng(x) = \sum_{n=0}^{\infty} b_n x^ng(x)=∑n=0∞bnxn, what is the coefficient of xkx^kxk in the sum f(x)+g(x)f(x) + g(x)f(x)+g(x)?
akbka_k b_kakbk
ak+bka_k + b_kak+bk
ak+bk2\frac{a_k + b_k}{2}2ak+bk
ak−bka_k - b_kak−bk